The ion exchanger has been widely used for a variety of applications in the form of films (i.e., in the form of ion-exchange films), such as a film for electrodialysis for concentrating the brine, desalting the saline water, and separating ionic substances from nonionic substances, and a diaphragm for electrolyzing alkali metal salt-containing aqueous solutions and organic solutions.
The above ion exchanger had heretofore been generally used in the form of flat films but, in recent years, has often been used in the form of tubes. When the ion exchanger is used in the form of a flat film, there occurs such an inconvenience that the ion exchanger occupies an increased installation area in the electrolytic cell (e.g., electroplating bath or electrodeposition bath). When used in the form of a tube, however, the ion exchanger occupies a decreased portion in the electrolytic cell. When used in the form of the flat film, further, ribs must be used to support the flat film to prevent the film from deforming, or gaskets or spacers must be used among the neighboring films resulting in a decrease in the effective film area that contributes to exchanging ions, or permitting foreign matters in the treated solution to adhere to the ribs, gaskets or spacers, arousing such problems that the blocking easily occurs and, hence, requiring the washing thereof and cumbersome disassembling operation. The tubular ion exchanger, on the other hand, requires no special member for supporting the ion exchanger or for preventing the deformation thereof and, therefore, maintains a sufficiently large effective ion exchange area offering such advantages as little blocking, easy washing and easy disassembling operation.
However, the tubular ion exchanger has a problem in that it is difficult to maintain both the mechanical strength and the electric properties. That is, the ion exchanger of this form is, usually, formed by using, as a starting material, a resin composition containing a granular ion-exchange resin and a thermoplastic resin that serves as a binder, and melt-extruding the resin composition. Therefore, the surfaces of the granular ion-exchange resin are covered with the thermoplastic resin without having ion-exchanging property and, therefore, the electric properties such as ion-exchange capacity, etc. are deteriorated (i.e., the film exhibits an increased resistance). To avoid such an inconvenience, the granular ion-exchange resin may be used in an increased amount and the thermoplastic resin which is the binder may be used in a decreased amount. In this case, however, the mechanical strength decreases and, for example, deformation is easily caused by fluid pressure or the like accompanied by such a problem that water permeates through during the use.
There has also been proposed a tubular ion exchanger maintaining both the mechanical strength and the electric properties. For example, a patent document 1 is proposing a tubular ion exchanger obtained by mixing a polyethylene type resin having a melt index of not more than 2 g/10 min. and a granular ion-exchange resin at a weight ratio of 3:7 to 5:5, and extrusion-forming the mixture into the form of a tube.